I originally posted this message in the all grain forum but I got a suggestion to post it here as well...

Many sources will match the bicarbonates in brewing water with the appropriate style of beer that can be brewed with that water. So in the case of a pale ale, you would want bicarbonate levels lower than 50 ppm or in the case of a stout, much higher, perhaps 200ppm. I've listened to waterganza on The Brewing Network and now all that makes perfect sense to me. Where it gets cloudy is when we talk about residual alkalinity. For example, take the water below, which is very close to my actual water. Using the "matching bicarbonates to beer style" example, having bicarbonates near 100ppm makes the water not so hot for a light colored pale ale. But, since my calcium is so low, I add about 6 or 7 grams of gypsum to my water. Now, using the "residual alkalinity" example, the RA is about 0, which makes my water good for a pale ale according to the nomograph John Palmer created. My confusion is this, I added calcium to the water which made the RA just right for my pale ale, but the 100ppm of bicarbonates are still there, which is not so good. What is the correct assumption....
a) Since the RA of my brewing water after adding gypsum is 0, this water is great for a pale ale
b) Since my water has 100ppm of bicarbonates, this water will never be suitable for a pale ale unless it's cut with RO water.

Adding enough gypsum (about 370 mg/L) to this water close to 0 RA would get you something that is fairly like Burton water from which you ought to be able to brew a pretty authentic Burton style pale ale. But you could brew a better pale ale by diluting with RO water to get the bicarb down and supplementing with calcium chloride to get the calcium up.

Part of "better" here means less hop harshness. If you disagree with that statement, i.e. if you like more assertive hop bitterness then supplement RO or RO diluted water with some gypsum in addition to the calcium chloride. The other part of better is less bicarbonate. The one rule of thumb in brewing water chemistry that I think may actually be valid is "alkalinity (bicarbonate) = bad".

A professional brewer would, in addition to decarbonating, add enough acid to the mash to get the pH into the 5.3-5.4 range.

The one rule of thumb in brewing water chemistry that I think may actually be valid is "alkalinity (bicarbonate) = bad".

Is that an "accross the board" rule? Meaning, is bicarbonate just as bad for stouts as it is for pale beers? I thought you needed alkalinity for dark beers to offset the acidity from the dark roasted grains?

Is that an "accross the board" rule? Meaning, is bicarbonate just as bad for stouts as it is for pale beers? I thought you needed alkalinity for dark beers to offset the acidity from the dark roasted grains?

That's a common misconception. In lab experiments I found that it takes 30% roast barley to pull the pH of a Maris Otter mash down to 5.2 using distilled water. 10% is more like the normal amount. If your water has 0, or negative RA and you use roast malts in that kind of concentration then perhaps you do need some bicarbonate. When I brew stout with water of nominal RA I get a pH higher than I'd like and should actually use some acid (but I don't - 5.5 is good enough though I'd prefer 5.4).

You should be ruled by your pH meter - not what a spreadsheet tells you.